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The Host-to-Host Layer Protocols

The Host-to-Host layerís main purpose is to shield the upper-layer applications from the complexities of the network. This layer says to the upper layer, ďJust give me your data stream, with any instructions, and Iíll begin the process of getting your information ready to send.ĒThe following sections describe the two protocols at this layer:

  1. Transmission Control Protocol (TCP)
  2. User Datagram Protocol (UDP)

Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) takes large blocks of information from an application and breaks them into segments. It numbers and sequences each segment so that the destinationís TCP protocol can put the segments back into the order the application intended. After these segments are sent, TCP (on the transmitting host) waits for an acknowledgment of the receiving endís TCP virtual circuit session, retransmitting those that arenít acknowledged. Before a transmitting host starts to send segments down the model, the senderís TCP protocol contacts the destinationís TCP protocol to establish a connection. What is created is known as a virtual circuit. This type of communication is called connection-oriented. During this initial handshake, the two TCP layers also agree on the amount of information thatís going to be sent before the recipientís TCP sends back an acknowledgment. With everything agreed upon in advance, the path is paved for reliable communication to take place.

TCP is a full-duplex, connection-oriented, reliable, accurate protocol, and establishing all these terms and conditions, in addition to error checking, is no small task. TCP is very complicated and, not surprisingly, costly in terms of network overhead. Since todayís networks are much more reliable than those of yore, this added reliability is often unnecessary.

User Datagram Protocol (UDP)

Application developers can use the User Datagram Protocol (UDP) in place of TCP. UDP is the scaled-down economy model and is considered a thin protocol. Like a thin person on a park bench, a thin protocol doesnít take up a lot of roomóor in this case, much bandwidth on a network. UDP also doesnít offer all the bells and whistles of TCP, but it does do a fabulous job of transporting information that doesnít require reliable deliveryó and it does so using far fewer network resources. There are some situations where it would definitely be wise for application developers to opt for UDP rather than TCP. SNMP monitors the network, sending intermittent messages and a fairly steady flow of status updates and alerts, especially when running on a large network. The cost in overhead to establish, maintain, and close a TCP connection for each one of those little messages would reduce what would be an otherwise healthy, efficient network to a dammed-up bog in no time. Another circumstance calling for UDP over TCP is when the matter of reliability is already accomplished at the Process/Application layer. Network File System (NFS) handles its own reliability issues, making the use of TCP both impractical and redundant. However, the application developer decides whether to use UDP or TCP, not the user who wants to transfer data faster.

UDP receives upper-layer blocks of information, instead of data streams as TCP does, and breaks them into segments. Like TCP, each UDP segment is given a number for reassembly into the intended block at the destination. However, UDP does not sequence the segments and does not care in which order the segments arrive at the destination. At least it numbers them, though. But after that, UDP sends the segments off and forgets about them. It doesnít follow through, check up on them, or even allow for an acknowledgment of safe arrivalócomplete abandonment. Because of this, itís referred to as an unreliable protocol. This does not mean that UDP is ineffective, only that it doesnít handle issues of reliability.

Further, UDP doesnít create a virtual circuit, nor does it contact the destination before delivering information to it. It is, therefore, also considered a connectionless protocol. Since UDP assumes that the application will use its own reliability method, it doesnít use any. This gives an application developera choice when running the Internet Protocol stack: TCP for reliabilityor UDP for faster transfers.

Key Concepts of Host-to-Host Protocols

Since we have seen both a connection-oriented (TCP) and connectionless (UDP) protocol in action, it would be good to summarize the two here. The following list highlights some of the key concepts that you should keep in mind regarding these two protocols.

 

TCP UDP
Sequenced Unsequenced
Reliable Unreliable
Connection-oriented Connectionless
Virtual circuit Low overhead

A telephone analogy might help you understand how TCP works. Most of us know that before you speak to someone on a phone, you must first establish a connection with that other personówherever they might be. This is like a virtual circuit with the TCP protocol. If you were giving someone important information during your conversation, you might ask, ďDid you get that?Ē A query like that is similar to a TCP acknowledgment. From time to time, for various reasons, people also ask, ďAre you still there?Ē They end their conversations with a ďgoodbyeĒ of some kind, putting closure on the phone call. TCP also performs these types of functions. Alternately, using UDP is like sending a postcard. To do that, you donít need to contact the other party first. You simply write your message, address the postcard, and mail it. This is analogous to UDPís connectionless orientation. Since the message on the postcard is probably not a matter of life or death, you donít need an acknowledgment of its receipt. Similarly, UDP does not involve acknowledgments.

 


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